The present invention relates to a directly-heated cathode structure and, more particularly, to a directly-heated cathode structure for use in a compact cathode ray tube which is utilized, for example, in the view-finder of a video camera.
In a conventional directly-heated cathode structure for a compact cathode ray tube which includes a pair of cathode lead pins, a pair of control (grid) electrode lead pins, a filament and an electron emitting material, the filament extends between the two cathode lead pins and is welded at each of the ends thereof to each of the leading ends of the cathode lead pins and the electron emitting material such as a metal carbonate (CaCO.sub.3, BaCO.sub.3, SrCO.sub.3) is applied to the filament at its approximate center portion.
A problem experienced in a conventional directly-heated cathode structure as explained above is that, since each end of the filament is welded while being pressed to each top end of the cathode lead pins, the former and the latter are not necessarily uniformly and evenly joined together. This is resulted from variations in the ways in which the ends of the filament are welded to the ends of the cathode lead pins. When a voltage is applied between the two cathode lead pins and the filament is heated for the emission of electrons from the electron emitting material, any defective weld or incomplete join in the respective ends may cause the filament to vibrate or render the filament to be easily affected by externally caused vibrations resulting in such problems as appearance of noise on the screen of the cathode ray tube due to instability in the emission of electrons.
Another problem in a conventional directly-heated cathode structure is that, when the electron emitting material is applied to the filament by a spray means, the electron emitting material scatters around so that the leading ends of the control electrode lead pins tend to catch the scattered electron emitting material, which makes it difficult to achieve a good welding of the control electrode to the leading ends of the control electrode lead pins.
The disclosure of a directly-heated cathode structure of the type explained above or at least a type similar thereto is found in, for example, Japanese Patent Application Kokai No. Sho 55-37,717 (1980), Hitachi Seisakusho K. K. as applicant; Japanese Patent Application Kokai No. Sho 57-36,750 (1982), Sony K. K. as applicant; and Japanese Patent Application Kokai No. Sho 57-87,041 (1982)-Kokoku No. Sho 63-53,661 (1988), Sony K. K. as applicant. It is to be noted, however, that none of these publications describes or illustrates the electron emitting material being applied to the entire length of the filament including its two end portions joining with the leading ends of the cathode lead pins, which is one of the features of the present invention.